A typical prior art milling cutter 10 having an axis of rotation A, and prior art cutting insert 12 for milling square shoulders in workpieces is shown in FIGS. 1 to 4. The prior art cutting insert 12 is generally rhomboidal in shape, having a top surface 14, a bottom surface 16, two major side surfaces 18 and two minor side surfaces 20. Some prior art inserts of this kind are also generally rectangular in shape. The two minor side surfaces 20 are provided with protruding portions 22. The major and minor side surfaces 18, 20 join the top surface 14 along a top edge 24, and the bottom surface along a bottom edge 26. FIG. 4 shows an illustrative bottom view of the prior insert of FIG. 1. The prior art milling cutter 10 has been removed so that the radial location of various parts of the prior art cutting insert 12 with respect to the axis of rotation A can be seen. The axial rake cannot be distinguished in this view, hence it appears that the axis A is parallel to the bottom surface 16 of the prior art cutting insert 12, but this is not the case, as can be seen in FIG. 1. As seen in FIG. 4, the top and bottom edges 24, 26 have a generally rhomboidal shape. The top edge 24 comprises two opposing major edges 28, constituting two major cutting edges, and two minor edges 30. The section of each minor edge 30 located on an associated protruding portion 22 constitutes a wiper 32. The bottom edge 26 has two opposing long edges 34 and two opposing short edges 36. The prior art cutting insert 12 has a basically positive geometry. That is, the included angle between the major and minor side surfaces 18, 20 and the bottom surface 16, taken between tangent planes to these surfaces at the bottom edge 26, is obtuse, whereas, the included angle between the major and minor side surfaces 18, 20 and the top surface 14, taken between tangent planes to these surfaces at the top edge 24, is acute.
As shown in FIG. 1 the prior art cutting insert 12 is seated in the milling cutter 10 with a positive axial rake; the axial rake angle being denoted by θ, with an operative major cutting edge 28′ positioned for peripheral milling. The other of the two major cutting edges 28 is not seen in the figures. The milling cutter 10 tapers forwardly at a forward portion 38 thereby giving it a conical shape. The forward portion 38 has a forward end 40 having a radius RA and a rear end 42 having a radius RB, where RB is greater than RA. The operative major cutting edge 28′ has a leading end 28′A adjacent the forward end 40 and a trailing end 28′B distal the forward end 40. As shown in FIGS. 2A, 2B and 3, the operative major cutting edge 28′ defines a cutting circle 44, which is included for reference. In FIG. 2A, the location of the rear end 42 of the milling cutter 10 has been marked with a dashed line showing the relative location of the forward and rear ends 40, 42 of the forward portion 38 relative to the cutting circle 44.
With reference to FIG. 4, it can be seen that when the cutting insert 12 is seated in the milling cutter 10, its bottom surface 16 has a forward radially outer bottom corner 46 and a rear radially outer bottom corner 48. It can also be seen that the operative major cutting edge 28′ in the region of its leading end 28′A is located at a greater radial distance from the forward radially outer bottom corner 46 than it is in the region of its trailing end 28′B from the rear radially outer bottom corner 48. Therefore, disadvantageously, the leading end 28′A of the operative major cutting edge 28′, which is subject to greater cutting forces than its trailing end 28′B, is provided with less support than the trailing end 28′B.
A milling cutter with a conical forward end is weaker than one with a cylindrical forward end. FIG. 3 shows a cylindrical milling cutter 50, that is, a milling cutter with a cylindrical forward portion 51. The cylindrical forward portion 51 has a forward end 52 having a radius RB equal to that of the rear end of the cylindrical forward portion 51 (not shown). If the prior art cutting insert 12 is seated in the cylindrical milling cutter 50, as shown in FIG. 3, then the forward radially outer bottom corner 46 of the prior art cutting insert 12 will be located at a radial distance RA. Since RA is smaller than RB the forward radially outer bottom corner 46 of the cutting insert 12 does not reach the periphery 53 of the cylindrical milling cutter 50. On the other hand, the rear radially outer bottom corner 48 does reach the periphery 53. Therefore, the long edge 34 of the bottom edge 26 joining the rear and forward radially outer bottom corner 48, 46 will not reach the periphery 53 of the milling cutter 50, apart from at the rear radially outer bottom corner 48. Therefore, the prior art cutting insert 12 will not be properly seated in the cylindrical milling cutter 50.
Clearly, the prior art cutting insert 12 has to be redesigned in order to be correctly seated. For a cylindrical milling cutter 50, designed to mill a square shoulder in a workpiece and for a cutting insert that is seated with a positive axial rake, the major side surfaces of the cutting insert will not be flat but generally twisted.